Conventionally, a method of manufacturing an electronic tube described in the Japanese Patent Application Laid-Open No. 59-94325 is known.
The configuration of an electronic tube for a television or a computer is shown in FIG. 1. The reference number 1 denotes a shadow mask, 1a an electron gun side surface of the shadow mask, 2 an electron gun, 3 a fluorescent screen, 4 electron beam, and 5 an electronic tube.
The shadow mask 1 formed of metal material is provided with a plurality of round or rectangular apertures on the whole surface so that electrons are projected only to desired micropositions on the fluorescent screen 3.
Although electron beam 4 irradiated from the electron gun 2 is projected to the whole shadow mask 1, only electrons passing through said apertures of the shadow mask 1 reach the fluorescent screen 3 to form an image.
Since the number of electrons colliding against the shadow mask 1 is sometimes larger than that of electrons passing through the apertures of the shadow mask 1, however, the kinetic energy of the colliding electrons is converted into thermal energy to elevate temperature of the shadow mask 1 to about 70.degree. C. or higher.
Thermal expansion of the shadow mask 1 following said elevation in temperature causes dislocation of the apertures of the shadow mask 1 and thus changes the positions of electrons irradiated on the fluorescent screen 3, leading to distortion of an image. Such a change in beam position caused by thermal expansion of a whole shadow mask, which results from irradiation of electron beam from an electron gun contained in an electronic tube to the whole surface of a fluorescent screen, is called "doming."
According to the conventional method of manufacturing an electronic tube 5, since an appropriate amount of coating of a paint containing a metal element with an atomic number not lower than 70 is considered to be not lower than 0.2 mg/cm.sup.2 but not higher than 2 mg/cm.sup.2 in order to suppress this doming, bismuth oxide powder and the like having a large effect of reflecting electrons (hereinbelow, referred to as electron reflection effect) has been painted on the electron gun side surface 1a of the shadow mask.
The electron reflection effect is known to be related to the atomic number of a material and to increase as the atomic number increases.
When material with a high electron reflection effect, such as bismuth oxide, is painted on the electron gun side surface 1a of a shadow mask, conversion of kinetic energy of electrons into thermal energy is prevented, since projected electrons are reflected by the surface 1a and do not enter the shadow mask 1.
Consequently, an elevation in temperature of the shadow mask 1 is prevented so that doming caused by thermal expansion can be suppressed and a problem of image distortion be solved.